1. Field of the Invention
The present invention relates to an apparatus and a method for determining a failure in a motor compressor system.
2. Description of Related Art
When a failure occurs in a part of a motor compressor system, for example, a compressor or a motor for driving the compressor, a current input to the motor changes. Therefore, it can be determined whether or not a failure in the motor compressor system has occurred by detecting the current and comparing the detected current with a predetermined limit reference current. In the case of a motor for driving a compressor., however, the current input to the motor changes significantly even though a failure does not occur, because compression work of the compressor will change in accordance with operating conditions.
Accordingly, when the limit reference current is fixed at a predetermined constant value, a problem arises in that there may be an indication that a failure has occurred, even though there actually has been no failure in the motor compressor system, if the limit reference current value is too low. On the other hand, where the limit reference current value is too high, there may be no indication that a failure has occurred, even though there has been a failure in the motor compressor system.
In order to solve this problem, in Japanese Utility Model Publication No. 3-97350, the present inventor disclosed a device for determining a failure in a motor compressor utilized for a refrigeration circuit. In this device, the limit reference current is changed on the basis of a discharge pressure of the compressor, a suction pressure of the compressor, a discharge temperature, or an evaporative temperature.
However, the current input to the motor for driving a compressor is increased in accordance with an increase of compression work of the compressor. In fact, actual compression work is increased in accordance with an increase of mass flow or an increase of compression ratio.
Therefore, in the determining device disclosed in Japanese Utility Model Publication No. 3-97350, a problem arises in that an accurate failure determination still can not be executed over the wide operation range of the compressor.